The hierarchical coding/decoding systems hierarchically organize the information to be transmitted or decoded from a digital signal in the form of a bit stream. Thus, according to the instantaneous bandwidth of the transmission channel or the processing capacity of the terminal reading the bit stream, all the stream, or only a part of the stream, is transmitted or decoded while ensuring that, in all cases, the essential information is transmitted and decoded.
These hierarchical systems also provide a differentiated channel protection of the data leading to a more robust transmission.
The current hierarchical audio coding techniques operate in frame-by-frame mode and the generated bit streams comprise access units describing the signal portions as indicated in the reference document relating to the “MPEG-4 audio” standard referenced ISO IEC SC29 WG11 International standard 14496-3:2001.
FIG. 1 shows a diagram of a bit stream 10 formatted from frames belonging to three levels 111, 112, 113 of a conventional hierarchical coding. The frames are therefore organized into a base layer 111 and two or more enhancement or enrichment layers 112 and 113 comprising frames 101 to 109 of the same duration.
For the construction of such a bit stream 10, only one strategy is conventionally considered. As illustrated by FIG. 1, the frames of the coded bit stream 10 are read according to the time axis t, then from the lowest level to the highest enhancement level (according to the axis Q), that is from the frame 101 to the frame 109.
The orders of priority of the frames are implicit.
The units are assigned a time stamp “cts” (standing for “Composition Time Stamp”). The two stamps correspond to the clock times by which the packets must be restored after decoding by the reading terminal.
Each unit with the same cts can be truncated (typically by a sending or routing device), the quality reconstructed on the decoder then being proportional to the number of layers received.
This conventional hierarchical coding/decoding technique considers only the transmission of entities for which the sending priority imposes a single hierarchy: either the units are of equal durations, or the base hierarchical level has a shorter duration than the other levels (example: enhancement of a CELP layer by a scalable AAC layer as stated in the reference document concerning the abovementioned “MPEG-4 audio” standard).